Slide ring seal

ABSTRACT

A shaft extending along and rotatable about an axis and a housing surrounding a portion of the shaft are provided with a seal assembly having an inner ring fixed on and rotatable with the shaft and having a pair of axially oppositely directed outer faces and a pair of outer slide rings axially flanking the inner ring and having inner faces axially sealed on the inner-ring outer faces. The outer rings, the housing, and the inner ring form an annular housing compartment surrounding the shaft. A blocking compartment on the housing is separate from the housing compartment. The inner ring is formed with a radially throughgoing passage having an inner end open on the shaft and an outer end open into the housing compartment. The housing is formed with a bore between the housing compartment and blocking compartment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT applicationPCT/EP2006/007253, filed 24 Jul. 2006, published 8 Feb. 2007 as WO2007/014666, and claiming the priority of German patent application102005036338.5 itself filed 29 Jul. 2005, whose entire disclosures areherewith incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a slide-ring seal, especially a slide-ring sealfor a housing, comprising an inner or counter-ring that can berotationally fixed on and sealingly engage a rotatable shaft, and twoslide rings between which the counter-ring is coaxially fitted, therings being provided with slide surfaces in such a manner that the ringscan rotate relative to each other in a sealed manner.

BACKGROUND OF THE INVENTION

Such slide-ring seals, that can also be designed as slide-ring seals fora housing, are generally known in the state of the art for sealing astationary element against a rotating element, customarily a shaft. Atypical area of application is for pumps that move a liquid. Aslide-ring seal is used e.g. at the position where the shaft is to besealed relative to the pump housing in order to drive a pump impeller incontact with a liquid to be transported. Accordingly, there is always aside in contact with a transport medium in a slide-ring seal.

Slide-ring seals are customarily designed in such a manner that aso-called counter-ring is rotationally fixed and especially sealed to arotary shaft. This forms a seal between the shaft and this counter-ringe.g. by at least one annular seal ring fitted to the counter-ring.

Such a counter-ring can be arranged between two so-called slide rings ofwhich one slide ring makes contact with the counter-ring via a slidesurface. This slide surface makes a sealed contact of the slide ring onthe counter-ring possible and at the same time permits rotation of theseelements relative to one another. The selection of the material forthese rings must be made accordingly. Usually, silicon carbide isselected as the material for the slide rings and also for thecounter-rings, which material has excellent running properties but hasthe disadvantage of being very brittle and therefore subject tofracture.

Thus, there is usually a leakage problem in such slide-ring seals in theregion of a seal ring that should ensure the seal between counter-ringand shaft, or, in the case of the rings consisting of a very brittle andtherefore breakable material such as for example silicon carbide.

If for example a seal ring on the counter-ring is defective, liquid canpass the slide-ring seal axially on the shaft and enter e.g. into themotor compartment of a pump, where it results in damage even before sucha leak is discovered. Furthermore, even fracture of one of the rings,especially of the counter-ring or of a ring inserted in it andconsisting e.g. of silicon carbide can result in a leak in which liquidcan penetrate through a slide-ring seal.

Rings, especially if they are manufactured from a very brittle materialsuch as e.g. silicon carbide can break on account of canting or also byin particular localized stress, especially when a motor is started e.g.when the rings run into a stop opposite a housing in order to ensurethat the unit consisting of slide rings and surrounding housing rotatesjointly relative to the counter-ring. High localized stresses of thering material and therefore also damage to the material can occurparticularly in the region of these rotation blockers.

The invention has the object of providing a new-design slide-ring seal,in particular a slide-ring seal for a housing, that makes it possible todetect a leak even before the fluid can pass from a medium side to thedry side where it can cause damage, and/or to prevent a leak, and whichpreferably has a lower susceptibility in particular to materialbreakage.

SUMMARY OF THE INVENTION

According to a first aspect of the invention this problem can be solvedin that at least one passage extending in particular radially isprovided in the counter-ring, one end of the passage facing the shaftand the other passage end opening in fluid communication with a blockingcompartment.

In the case of a leak between counter-ring and shaft, that is,substantially axially, it can either be ensured in such a manner thatleak fluid is moved out through the passage in the counter-ring and thuscannot go axially through the slide-ring seal, and/or that leak fluid isprevented from passing through the seal in that a blocking compartmentin fluid communication with the passage is placed under pressure and inthis manner the leak fluid is forced back by the blocking fluid in theblocking compartment. Thus, in both instances the leak fluid cannot getthrough, that is e.g. to the fluid being moved by a pump.

The form and in particular the cross section of such a cited passage canbe as desired. The passage can be formed in a simple manner as a borewith a round cross section. Several passages are preferably providede.g. uniformly angularly distributed around the shaft.

In a preferred further development a passage end opening, facing theshaft, of the passage can be arranged in a region between two coaxiallysuccessively arranged sleeve/seal rings in the counter-ring.

This design of a slide-ring seal with two or more coaxial, successivesleeve/seal rings in the counter-ring that are spaced from each otherand that seal the counter-ring against the shaft, ensures that upon theloss of the sealing function of one of the two sleeve/seal rings thefluid running in an axial direction through the defective seal passesinto the open region of at least one passage between two sleeve/sealrings and then through the passage into the blocking compartment or aninner compartment of the slide-ring seal for the housing communicatingwith the blocking compartment or is forced back by pressurized blockingfluid.

This also provides redundancy in the sealing of the counter-ringrelative to a shaft since now at least two sleeve/seal rings are used.If a fatigue phenomenon occurs that can result in a leak in one of thesleeve/seal rings, a seal still is formed because at least one furtherseal ring is left so that an axial leak is impossible. It can optionallybe provided here that more than two sleeve/seal rings are used in theregion of the sealing surface between the counter-ring and the shaft.

Each of these individual seal sleeves or can be held e.g. fitted in theslide ring in an annular groove provided for this purpose and extendoutward somewhat beyond the inner surface of the counter-ring.Accordingly, if a slide-ring seal with such a counter-ring is placed onthe step provided for this purpose on the shaft, the sleeve/seal ringsare radially compressed and form a tight, in particular rotationallysecure seat of the counter-ring on the shaft. A latch or rotationblocker of the counter-ring can be supported even more by furthermeasures that are familiar to an expert in this art.

A preferred further development can provide that a slide-ring seal ofthe type in accordance with the invention comprises sensor means withwhich a leak of the slide-ring seal, in particular in the region of thecounter-ring, can be detected, in particular in such a manner that aleak axially through the passage can be detected in the case of a defectof the sleeve on the product side.

The sensor means can be designed in such a manner that it reacts to thefluid that is to be sealed by the slide-ring seal against anenvironment. For example, in the case of water or many other liquids thesensor can detect electrical conductivity when such a fluid makescontact with the sensor. A sensor can therefore be formed e.g. byelectrodes. There is likewise the possibility of arranging other sensorsadapted to the particular fluid in such a slide-ring seal. In thismanner a leak can be determined very early, especially before the fluidpenetrates through the slide-ring seal.

In a preferred further development at least one sensor of the sensormeans can be arranged in a blocking compartment and/or in regionscommunicating with a blocking compartment by means of which the presenceof fluid, especially fluid being moved by a pump, can be detected. Itcan be provided in this instance that the blocking compartment and/orregions communicating with the blocking compartment is/are filled with ablocking fluid, especially with medicinal white mineral oil or anon-oxidizing liquid that evaporates in a residue-free manner.

Thus, if in such an instance a sensor is insensitive to the blockingfluid used but it is nevertheless permeated by leak fluid, detectiontakes place and service measures can be initiated. It can preferably beprovided that the blocking fluid can be replaced.

A counter-ring arranged on a shaft in a fixed and rotation-blockedmanner is frequently manufactured completely from a suitable materialsuch as e.g. silicon carbide, so that there is the danger that if thematerial is damaged e.g. by breakage, this basically has a global effecton the ring in its totality and can thus result in a leak. It is alsoknown that a slide ring can rest via at least one support ring on thecounter-ring, in particular when a support ring is fitted in an annulargroove open in particular axially on the counter-ring.

By means of this measure the slide ring and the support ring can beoptimally coordinated with one another as regards their material inorder to ensure the optimal tightness, service life and freedom ofrotation, in which case the counter-ring, in which the two support rings(at least one for each support ring) are arranged in a fixed manner, canbe manufactured from a different material that has a higher resistanceto stresses.

Thus, in this construction in accordance with the invention thecounter-ring can be manufactured e.g. from metal and the support ringsmanufactured e.g. from silicon carbide or some other suitable materialwith known excellent properties of with regard to rotation and sealing.Such a counter-ring therefore forms a unit with the support rings.

It is important in this construction that a support ring be arrangedtightly and also fixedly in the counter-ring. This can take place bymeasures known to an expert in the art e.g. by an gluing or shrinking inplace. Furthermore, the receiving groove into which a support ring isinserted on the counter-ring can be designed slightly larger in itswidth that the radial thickness of the support ring in order ensure thatthe latter comes sits without tension in the counter-ring.

This construction has the advantage that given a mechanical stress and apossible fracture of the support ring the entire counter-ring is notcompletely ruined immediately but rather the latter continues to remainon the shaft in a secure and fixed manner and thus a leak axiallycontinues to be effectively prevented.

Only a leak substantially radially, that is, substantially parallel tothe sliding surface between slide ring and support ring can occur inthis instance so that any leakage liquid penetrates at first into theslide-ring seal or into the housing or the blocking compartment and anassociated region of the slide-ring seal where it continues to beretained, is forced back by blocking fluid and/or is detected by asensor.

This takes place in an especially advantageous manner if a support ringborders at least partially the blocking compartment and/or regionscommunicating with the blocking compartment so that in case of a leakthe leaking fluid penetrates directly into these regions.

The blocking compartment can be arranged radially at least outside oneof the rings, that is, around one of the slide rings or around thecounter-ring, and in an especially preferred embodiment the blockingcompartment itself can be annular and coaxially surround at least one ofthese rings, optionally all the rings of the slide-ring seal. Thisensures that in the case of a leak between the rings in the sealingregion and/or sliding region any penetrating fluid can pass into theblocking compartment and be detected there.

Such a blocking compartment can be realized in an especially simplemanner in its design in the case of a so-called glide-ring seal for ahousing, in which seal the counter-ring and slide rings are covered, inparticular enclosed by a housing coaxially annularly surrounding therings, in which case the housing rests in a sealing manner with radiallyinwardly located surfaces on radially outwardly located surfaces of theslide rings. This ensures that the entire arrangement consisting ofhousing and slide ring and counter-ring forms a tight unit that can bepushed onto a shaft. It is especially advantageous in this case from amanufacturing standpoint if the housing is divided axially into twoparts, especially symmetrically, that can therefore be made as twohousing halves fitted over the glide rings and then connected to eachother. This can take place by retaining pins provided for this purpose.

In a preferred variant of such a slide-ring seal the arrangement ofhousing and the rings forms an enclosed, annular inner compartmentinside this slide-ring seal that can either form the blockingcompartment itself or communicate especially via at least one bore inthe housing with a blocking compartment arranged e.g. around it.

For the case where the annular inner compartment is not to form theblocking compartment itself, a blocking compartment that is inparticular annular can be arranged in a sealing manner on a surface ofthis housing, which is located radially on the outside e.g. by screwing.

Accordingly, in a slide-ring seal designed in this manner if there is abreak of one of the support rings between the slide ring and thecounter-ring or if there is a leak in the region of a seal/sleeve ringbetween the counter-ring and the shaft the fluid will first pass througha support ring or through a passage in accordance with the inventioninto the inner compartment of the formed slide-ring seal of the housingand either be directly detected there in the inner compartment or thefluid can pass through passages extending from the inner compartmentthrough the outer housing wall into a blocking compartment locatedoutside it.

For a better extraction of heat, especially in the region of the slidesurfaces, the slide-ring seal can comprise as a supplement a transportdevice for fluid, in particular by fins/blades (16) that are arranged ina radially projecting manner and are in particular axially aligned onthe counter-ring (10) and surrounded by blocking fluid. The improvedthermal transport on the glide surfaces can bring about lesser wear.

Furthermore, the counter-ring seal forms a tight unit that is closedwithin itself so that at least in the case of a leak the entiretightness continues to be retained but the leak can be detected so thatthe apparatus in which such a glide ring seal is being used can beserviced.

Thus, all the above-described embodiments, in particular in anespecially preferred combination of all these embodiments but also evenwhen optionally used alone, achieve the considerable advantage that inaddition to redundancy on account of the doubling of the relevant partsat the same time a reliable detection of all possible and of the mostprobable leakages can also take place.

It can be provided in all the previously cited alternatives orembodiments to be combined with each other in a cumulative manner thatthe slide rings are pressed against the counter-ring or against thesupport rings arranged in it on account of a spring acting between thehousing and the slide ring, in particular in such a manner that axialplay remains between the housing and the rings. To this end e.g. aspring can be arranged between a radially inwardly projecting region ofa housing and a radially outwardly projecting region on a slide ring sothat the spring is arranged with its axis substantially parallel to theaxis of a shaft. Axial shaft movements, in particular when starting anelectric motor, can thus be detected.

In addition, a slide-ring seal can cant in its mounts, especiallybetween the housing and slide ring, at an angle to the axis of rotation,as a result of which high localized stresses can occasionally beproduced in the partially brittle material of the slide rings in knownprovided rotation blockers.

Thus, it is known e.g. in the state of the art that rotation blockingcan be achieved in that axially aligned pins are arranged on a housingand cooperate with a stop on a slide ring in order to prevent rotationof these elements relative to one another. However, only linear contactand in the case of a canting even only a point contact can occur betweenthe surface of such an axially aligned pin and a stop on the slide ring,so that the stresses are particularly high here, especially whenstarting a motor, and accordingly can result in damage.

In order to ensure greater reliability here it can be provided inaccordance with a second aspect for solving the problem in a design of aslide-ring seal in accordance with the invention and of the previouslycited generic type in which the counter-ring and the slide rings arecovered, especially covered over, by a housing arranged annularly andcoaxially to the rings, that the rotation blocker comprises at least onepin on which a stop element with a stop surface is rotatably arranged,which stop surface cooperates with an abutment. Such a slide-ring sealcan also be combined as regards its features with all previously citedembodiments.

This brings it about in especially advantageous manner that on the onehand the abutment is localized in any case and that on the other hand asa consequence of the freedom of rotation of the stop element on a pinits stop surface is always automatically aligned with the abutment oncontact so that an surface contact is produced between the stop and theabutment. This results in lesser stressing and therefore in less damage.It can be provided here that the freedom of rotation is possible only ina certain angular range. Rotation is preferably always less than 360°and is limited e.g. by the surrounding structural elements.

It can be provided in a first embodiment that the pin is fixed on thehousing in particular perpendicular to the axial direction of theslide-ring seal and that the abutment is provided on a slide ring, inparticular on a radially outwardly facing protection of the slide ring,which projection comprises an abutment surface. As a consequence of thisconstruction the pin is aligned e.g. perpendicular to the axialdirection of the slide-ring seal and the stop element can rotate withits stop surface about the axial direction of the pin, and it is ensuredthat even in the case of canting at an angle to the axis of rotation ofthe slide-ring seal an surface contact is achieved.

In a kinematic reversal of the above-cited embodiment it can also bealternatively provided that the pin is provided on a slide ring, inparticular perpendicular to the axial direction of the slide-ring seal,and that an abutment is provided on the housing, in particular on aradially inwardly facing projection comprising an abutment surface.

It is a significant viewpoint for the advantageous design that one ofthe two stops, that is, either the stop surface of the previously citedstop element or even the stop surface of the previously cited abutmentcan move at least within limits relative to the structural component towhich the stop is fastened in order to be able to align itself relativeto the particular other stop in this manner upon a possible canting.This is achieved as previously stated preferably by means of a rotationabout the pin axis.

In an especially advantageous, constructively simple embodiment a pincan be designed to this end e.g. as a pin with external screwthread anda stop as a polygonal nut with internal screwthread. Accordingly, thepolygonal nut can be screwed onto the pin, which polygonal nut hasseveral surfaces on its outer side that correspond to the number ofedges, which surfaces can be aligned at least in one dimension about thepin axis in accordance with the possibility of the rotation of thepolygonal nut in order to fit with the respective other stop in thismanner.

In an embodiment of the abutment in the counter-ring the counter-ringcan comprise e.g. a radially projecting collar comprising a slot or anaxial groove and the width of the slot or groove is dimensioned in sucha manner that the stop element can fit in it. As a result thereof andalso by means of an appropriately small spacing of a rotatable stopelement an angular limitation of the freedom of rotation can beachieved.

BRIEF DESCRIPTION OF THE DRAWING

An illustrated embodiment of the invention is presented in the soleFIGURE of the drawing and discussed in the following.

SPECIFIC DESCRIPTION

The drawing shows a cross-section of a slide-ring seal for a housingwhere a seal is fitted on a shaft W that can here rotate about an axisA. The slide-ring seal is pressed against the surface of the shaft W byan inner ring or counter-ring 10 in such a manner that the counter-ring10 is rotationally fixed to and sealed against this shaft W. The sealbetween the counter-ring 10 and the shaft W is achieved by means of twospaced and coaxial seal rings 5 made e.g. of an elastomeric material.

The counter-ring 10 is fitted between two outer slide rings 1 and 1 athat bear on the counter-ring 10 via support rings 4 with their slidefaces 1 b, which support rings 4 are fitted e.g. shrunk, into coaxiallyopen grooves of the counter-ring 10.

These grooves are each slightly larger than the radial thickness of therespective support ring 4, as can be recognized here, in order ensurethat these rings 4 can be fitted into the grooves without stretching.

It can be furthermore recognized in this embodiment that the slide rings1 and 1 a as well as the counter-ring 10 are enclosed by a coaxialhousing consisting of two housing halves 6 and 7 forming an innercompartment 11.

A respective spring 3 is braced between a radially outwardly projectingelement on each of the slide rings 1 and 1 a and a respective radiallyinwardly projecting element (e.g. collars running around the shaft) onthe housing halves 6 and 7, by means of which each slide ring 1 or 1 ais biased axially inward toward the counter-ring 10 or the respectivesupport ring 4 in order to ensure a tight seat at the respective slidesurface 1 b. Here, the two housing halves 6 and 7, which can be securedto each other by retaining pins 8, are each sealed to the respectiveslide ring 1 or 1 a via a respective O-ring seal 2 fitted in a groove inan outer surface of the respective slide ring.

The housing slide-ring seal shown in FIG. 1 has the particular advantageon the one hand of redundancy on account of there being two seal rings 5and of the provision of respective support rings 4 on the counter-ring10.

In this case, the counter-ring 10 can for example be made from metal andthe support rings 4 can be made of silicon carbide or some othermaterial capable of sliding that works especially well with the materialof the slide rings 1 and 1 a, which can also be silicon carbide.

In case of damage, failure of one of the ring seals 5 will notinevitably lead to failure of the other ring seal 5, that is only onewill fail. Accordingly because the other seal is still undamaged noliquid can move axially along the shaft under counter-ring 10.

In order to make it possible to detect such a leak here, thecounter-ring 10 is formed with a passage 12 that here extends radiallyand empties on the one hand into the region between the sleeves and onthe other hand into an inner compartment 11. Liquid that accordinglygets past the counter-ring on account of a leak in a seal ring 5 istherefore conducted through the passage 12 into the inner compartment11, in which a sensor 20 is already optionally arranged in order todetect this liquid. Also, the inner compartment 11 can be pressurizedwith a blocking fluid and thus force the leak fluid back.

Furthermore, it also possible that, as indicated here, the two housinghalves 6 and 7 are formed with radially throughgoing bores 13 via whichthe inner compartment 11 can communicate with a further ring elementthat is tightly secured via O-rings 9 to the outer surface of thehousing 6, 7 to form a blocking compartment 19 in the sense of theabove-described invention. Thus, the sensor such as shown at 20 fordetecting leakage can also be housed in this blocking compartment 19,and all the inner compartments, that is the blocking compartment 19, thecommunicating passages 13, the inner compartment 11 and the passage 12are filled with a blocking liquid different from the medium that theseal is against. This can be e.g. oil or even pure water.

In another instance of leakage e.g. a support ring 4 can break due toits brittleness e.g. when formed from silicon carbide. If this should bethe case, the fracture of such a ring 4 will not also extend to theother ring 4 because of the separation of material and also not to thecounter-ring 10 since all these structural components are independent ofeach other even though they are connected together.

Accordingly, in the case of a break or a leak in one of these rings 4 noleakage liquid can likewise travel axially through the slide-ring sealbut instead leak liquid will pass through the damaged support ring 4substantially radially back into the inner compartment 11 and beimmediately detected there, or, after having been conducted farther bythe passages 13, inside the blocking compartment 19 not shown here.

This ensures that in the case of the most likely occurring leakages theslide-ring seal continues to remain tight as a unit on account of theincreased redundancy and on the other hand detection of the leak canalso take place before damage to the apparatus occurs in which the glidering seal is being used.

Radially projecting and in particular substantially axially extendingfins 16 can be formed on the counter-ring 10 on its outer side facinginto the inner compartment 11. These fins 16 form scoops on thecounter-ring 10 and put the fluid in the inner compartment 11 or theblocking compartment 19 into motion. This can achieve a more rapidtransfer of heat, particularly in the region of the slide surfaces 1 b,and thus lower wear.

Furthermore, FIG. 1 also shows a design of a rotational coupling in theright region that is arranged here for realization in each housing half6 and 7 in order to prevent that either of the two slide rings 1 canrotate relative to the housing. Thus, the entire unit consisting ofslide rings and housing will rotate relative to the counter-ring whenthis rotation blocker is used.

In order to achieve this, threaded pins 14 are arranged radially andtherewith substantially perpendicular to the axis of rotation A insidethe housing so that their ends project into the inner compartment 11. Apolygonal, here square, nut 17 is screwed onto the inner end of eachthreaded pin 14. This nut is axially fixed on the respective pin 14 byits screwthread but can rotate about the axis of pin 14 at least limitedangularly and cooperates with an abutment 18 mounted on a radiallyoutwardly directed projection on the respective slide ring.

This ensures that if the ring arrangement should cant relative to theaxis of rotation A outside of the view plane considered here, the twostop regions, that is, the surface of a nut 17 and the particularabutment 18 continue to rest against each other in surface contact andthus load peaks are avoided.

Accordingly, all measures of the invention suggested here contribute toan increased safety, longer service life and to better maintenance.

1. In combination with a shaft extending along and rotatable about anaxis and a housing surrounding a portion of the shaft, a seal assemblycomprising: an inner ring fixed on and rotatable with the shaft andhaving a pair of axially oppositely directed outer faces each formedwith a respective radially outwardly open groove; a pair of outer sliderings axially flanking the inner ring and having inner faces axiallysealed on the inner-ring outer faces; respective support rings in thegrooves of the inner-ring outer faces and engaging the outer-ring innerfaces, the outer rings, the housing, the support rings, and the innerring forming an annular housing compartment surrounding the shaft; aliquid in the compartment; radially projecting vanes in the housingcompartment on the inner ring, whereby as the inner ring rotates in thehousing compartment its vanes move the liquid therein; a respectivegroove having axially spaced flanks and open radially outwardly into thehousing compartment on each of the outer rings; respective radiallyinwardly projecting elements fixed in the housing and each having athreaded inner end in the compartment aligned radially with a respectiveone of the grooves; and respective nuts threaded and rotatable on theinner ends, received in the respective grooves of the outer rings, andangularly engageable with the flanks of the respective grooves of theouter rings for preventing free radial rotation of the outer rings inthe housing.
 2. The seal assembly defined in claim 1, furthercomprising: a blocking compartment on the housing separate from thehousing compartment, the inner ring being formed with a radiallythroughgoing passage having an inner end open on the shaft and an outerend open into the housing compartment, the housing being formed with abore between the housing compartment and blocking compartment.
 3. Theseal assembly defined in claim 2, further comprising two annular shaftseals axially spaced from each other and each radially inwardly engagingthe shaft and radially outwardly engaging the inner ring, the innerpassage end opening radially inward on the inner ring between theannular shaft seals.
 4. The seal assembly defined in claim 3, furthercomprising: sensor means for detecting liquid in one of thecompartments.
 5. The seal assembly defined in claim 4 wherein the sensormeans is in the housing compartment.
 6. The seal assembly defined inclaim 4 wherein the sensor is in the blocking compartment.
 7. The sealassembly defined in claim 2, further comprising a liquid filling theblocking compartment that is different from a liquid lubricating theshaft.
 8. The seal assembly defined in claim 7 wherein the liquid in theblocking compartment can be exchanged.
 9. The seal assembly defined inclaim 1 wherein the grooves of the inner rings have a radial widthgreater than a radial thickness of the respective support rings so thatthe support rings are seated with radial play in the respective grooves.10. The seal assembly defined in claim 1 wherein the support rings areof silicon carbide.